This invention relates generally to the field of digital cameras, and in particular, to a mount for aligning an image sensor to an optical axis of the digital camera.
High resolution digital cameras have been developed for professional photographers, whose business depends on taking high quality pictures. These cameras often use existing 35 mm or medium-format film camera bodies and lenses, together with a customized electronic imaging back that replaces the removable film back ordinarily supplied with the camera. An example is the Kodak DCS 460(trademark) digital camera, which is based upon the Nikon N90s single lens reflex (SLR) camera body. This system, which is compatible with all Nikon lenses and accessories, allows photographers to easily operate the digital camera, and to use their existing equipment with the camera. However, the photosensitive area of the image sensor is often smaller than the film area, and therefore, smaller than the image plane. As a result, the relative magnification of the lens is larger than for 35 mm film.
A problem with this type of camera is the alignment of the image sensor to the optical axis of the camera body. For a non-SLR camera, where the image sensor may be an integral part of an optical assembly, the alignment technique in Japanese Patent Application 61-245773 can be used. Two positioning pins are provided to the rear end of a lens barrel, and are pressed into contact with a ceramic package containing the image sensor in order to optically position the image sensor at an image plane of a lens assembly. However, in an SLR camera system, the image sensor cannot physically locate to the lens assembly because the lens, which is removable, is separated from the image plane by a movable mirror used for through-the-lens viewing.
Consequently, in a camera back system using a film body, the image sensor needs to be referenced to the film plane. For example, as shown in U.S. Pat. No. 5,483,284, an image sensor package possesses four positioning pins at four corners thereof that touch a guide rail plane of an aperture in the camera body. With this arrangement, the image sensor package is precisely located at the focal plane of the lens assembly, i.e., in the z-direction. However, it is difficult to prevent x-y misalignment, which in turn prevents accurate centering of the image sensor package relative to the optical axis of the lens assembly.
An imager mounting plate can also be used to support the image sensor against the camera guide rails at the focal plane of the lens assembly. The imager mounting plate can then be secured to the camera guide rails using a pin and hole configuration to provide x-y alignment. However, the camera guide rails are typically made of either machine steel or molded plastic. Providing alignment holes in machine steel rails would result in increased manufacturing costs. Plastic molded rails, although less expensive to manufacture, do not provide sufficient accuracy of alignment of the image sensor at the focal plane of the lens assembly, and do not allow for interchangeability of the image sensor for servicing. Further, in a camera back system using a film body, warping or deflections in the camera guide rails or in the imager mounting plate can often cause distortions in the digital images.
Accordingly, it is an object of the present invention to provide an image sensor mount for a digital camera for accurate alignment of the image sensor at the focal plane of the lens assembly.
It is another object of the present invention to provide an image sensor mount for a digital camera which enables ease of replacement and serviceability of the image sensor without using camera guide rails.
It is yet another object of the present invention to provide an image sensor mount for a digital camera which substantially removes any distortions in the digital images.
It is a further object of the present invention to provide an image sensor mount for a digital camera which eliminates the need for mounting holes in the camera housing.
These objects are achieved in a digital camera having an image sensor and a lens defining an optical axis for focusing an image upon a focal plane, an image sensor mount comprising:
(a) the digital camera having a housing for mounting the lens and defining a first opening through which an image can be focused by the lens and transmitted along the optical axis;
(b) an image sensor assembly including the image sensor attached to an image sensor mounting plate;
(c) a camera mounting plate having first and second surfaces, the first surface being fixedly attached to the camera housing and defining a second opening through the first and second surfaces which is aligned with the first opening when the camera mounting plate is mounted to the camera housing, the camera mounting plate including at least three spaced-apart alignment elements on the second surface, the alignment elements being located at a predetermined position relative to the lens; and
(d) the image sensor mounting plate being attached to the camera mounting plate so that the image sensor mounting plate engages the alignment elements to center the image sensor with respect to the optical axis at the focal plane.
It is an advantage of the present invention to provide an image sensor mount for a digital camera which enables interchangeability of the image sensor for servicing, while maintaining proper focus alignment accuracy.
It is another advantage of the present invention to provide an image sensor mount for a digital camera which does not introduce any distortions in the digital images.
It is yet another advantage of the present invention to provide an image sensor mount for a digital camera which does not require the use of camera guide rails or of mounting holes in the camera housing, thereby reducing manufacturing cost and complexity.